Preamplifier fly height control (FHC) driver and sensing circuit

ABSTRACT

Managing temperature of a read/write head ( 120 ) in a disk drive system in which there is a power variance due to different operation modes. A circuit device ( 100 ) determines and delivers additional power needed for compensating for the temperature variance due to different operational power requirements. The power is delivered to a resistive heater (Rheat) associated with the head ( 120 ). The compensating power is based on the delivery voltage, delivery current, and resistance of the resistive heater (Rheat). The delivery current is varied to account for changes in the resistance of the resistive heater (Rheat) since it can vary with temperature. By sensing the current with a sensor ( 13 ), the resistance is determined via the sensed current and the delivery voltage. The current is adjusted for maintaining the compensating power.

FIELD OF THE INVENTION

The invention relates generally to control devices and, moreparticularly, to a control driver for disc drive systems.

BACKGROUND OF THE INVENTION

In disc drive systems, the discs are mounted on a hub of a spindle motorfor rotation at an approximately constant high speed during theoperation of the disc drive. An actuator assembly in the disc drivemoves magnetic transducers, also called read/write heads, to variouslocations relative to the discs while the discs are rotating, andelectrical circuitry is used to write data to and read data from themedia through the read/write heads. The fly height, also calledclearance, is a distance between the read/write head and the media.

Disc drives are being produced with increasing track densities anddecreasing access times. A read/write head must fly over the media of adisc as closely as possible to improve reading and writing access times.Further, the fly height of the head should be approximately uniform fromread mode to write mode and during the reading and writing to improvesystem performance.

Several variables can affect the fly height of a head. For example, flyheight is impacted by a curvature of a disc, vibrations of the disccaused by the spindle motor, and roughness and defects in the media. Flyheight is also affected by variation in the heat dissipated in the headdue to the differential in the power characteristics of the head whilein the read mode versus the write mode. For example, heat causes theread/write head to expand. As more power is delivered to the head, thehead tends to expand more. Therefore, the head will tend to expand moreduring a reading or writing operation as the head heats up. The headwill also expand more during the write mode since it requires more powerto write than read. The disturbance decreases performance and/orincreases the possibility of an error in reading from or writing to themedia.

Several efforts have been made to improve control of the fly height of aread/write head. However, none of the efforts have resulted in asuitable solution to the aforementioned heat disturbance problem. Thereremains a need for a system to control the fly height of a read/writehead to allow it to read data from or write data to closely spacedmedia.

SUMMARY

The present invention achieves technical advantages as a method,apparatus and system for managing temperature variations in anelectronic device. For example, managing temperature variations in adisc drive read/write head which are due to power variance of the readand write modes. Additional power needed for compensating for thetemperature variance due to different operational power requirements isdetermined and delivered for resistive heating, for example, in aresistive heater to increase temperature. The resistive heater is in aheat transfer relationship with the read/write head such that the heatdue to the delivery of additional power to the heater is transferred tothe head. The compensating power is based on the delivery voltage,delivery current, and resistance of the resistive heater. The deliverycurrent is varied to account for changes in the resistance of theresistive heating since it can vary with temperature. By sensing thecurrent, the resistance can be determined via the sensed current and thedelivery voltage. The current is adjusted for maintaining thecompensating power.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference ismade to the following detailed description taken in conjunction with theaccompanying drawings wherein:

FIG. 1 illustrates a read/write driver system in accordance withexemplary embodiments of the present invention;

FIG. 1A illustrates a control schematic in accordance with exemplaryembodiments of the present invention;

FIG. 2 illustrates a control circuit in accordance with exemplaryembodiments of the present invention; and

FIG. 3 illustrates a control circuit in accordance with exemplaryembodiments of the present invention.

DETAILED DESCRIPTION

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferred exemplaryembodiments. However, it should be understood that this class ofembodiments provides only a few examples of the many advantageous usesand innovative teachings herein. In general, statements made in thespecification of the present application do not necessarily delimit anyof the various claimed inventions. Moreover, some statements may applyto some inventive features, but not to others. Throughout the drawings,it is noted that the same reference numerals or letters will be used todesignate like or equivalent elements having the same function. Detaileddescriptions of known functions and constructions unnecessarilyobscuring the subject matter of the present invention have been omittedfor clarity.

The following inventive embodiments are described in terms of a discdrive system, however, the invention can also be used in other systemsor devices in which heat control to an electronic element can improveperformance. Referring now to FIG. 1 there is illustrated a disc drivesystem in accordance with exemplary embodiments of the presentinvention. The system includes a preamplifier 100 coupled with aread/write head 120 along an extended arm (not shown) reaching adjacenta magnetic storage disc. The head 120 is typically suspended on theextended arm in close media proximity with media 130. Media 130 istypically a magnetic storage disc. The preamplifier 100 includescircuitry for applying both read and write signals to the head 120. Thepreamplifier 100 is also adapted to determine the signal power for eachof the read and write modes.

Because of the variance in power delivered to the head 120 from the readmode to the write mode, there results a corresponding variance in theheat delivered to the head 120. According to exemplary embodiments ofthe present invention, the preamplifier 100 is used to manage heating ofthe head 120 such that the heat is maintain at a constant value despitevariable heating resulting from variable power delivery to the head 120for the different operation modes. The preamplifier 100 determinesadditional power requirements needed for compensating for thetemperature variance due to different operational power requirements(i.e., reading and writing modes) and delivers the appropriate amount ofheat to the head 120 (via resistive heating, for example). Thepreamplifier 100 can include a resistive heater arranged in a heattransfer relationship with the read/write head 120 such that heat fromthe resistive heater due to the delivery of the compensation power tothe heater is transferred to the head 120.

The compensating power is based on the delivery voltage, deliverycurrent, and resistance of the resistive heater. The preamplifier 100 isoperable for varying the delivery current to account for changes in theresistance of the heater since the heater's resistance can vary withtemperature. For detecting the resistance of the heater, thepreamplifier 100 includes a sensor for sensing the current delivered anddetermining the resistance of the heater based on the delivery voltage.

Referring now to FIG. 1A there is illustrated a circuit representationof a preamplifier 100 in accordance with exemplary embodiments of thepresent invention. The preamplifier 100 includes amplifier 11, feed backwith resistors Rf, sensing device 13, and a heater, such as a heatelement resistor (Rheat). The heater Rheat is in a heat transferrelationship with the head 120 and/or is integral to the head 120. Thepreamplifier 100 is cooperable with the operational power deliverycircuits for the read and write mode such that approximately the sametotal power is delivered to the head 120 during both the read and writemodes. Thus, the heat to the head 120 is managed by determining thepower delivered to the head 120 for reading and writing and furtherdelivering additional power to the heat resistor Rheat as needed tocompensate for any head heat variances.

Since a heat resistor typically possesses a large temperaturecoefficient which causes the heater's resistance to vary depending onthe power delivered to the heater, it is necessary to measure thecurrent through the heat resistor in order to calculate the resistanceand make appropriate adjustments to maintain power delivery to the head.However, it is also advantageous that the measurement circuitry notimpede the FHC driver's ability to deliver maximum power. The amplifier11 is used to accurately drive the heat resistor Rheat, whereVout=2×Vref. Vref is the reference voltage that can be programmed. Thefeedback through resistive divider Rf with gain of 2× not only ensuresthe accuracy of Vout, it also facilitates the Vref generator designsince it does not need to be driven as high as Vout which needs to goclose to the positive power supply voltage (VCC). By scaling the gain onthe feedback, an amplifier can be used which does not requirerail-to-rail voltage.

With a constant applied voltage of the present preamplifier 100, theheater resistance is determined by sensing the corresponding appliedcurrent with sensing device 13. The sensing device 13 can constantlysense the current such that the resistance can be monitored over time.The sensing device 13 includes an output for outputting the sensedcurrent to control devices for corrective adjustments as needed, forexample in the event the resistance of the heater increases. Further tonot degrade the driver's performance, the sensing device 13 produces asensed current value (Is) which is only a small proportion of the actualcurrent delivered to the heat resistor (Rheat).

In exemplary embodiments a vertical PNP or PMOS is used as the outputdevice to deliver Vout. Therefore, the maximum Vout value would then beVCC−Vce for PNP, or VCC−Vds for PMOS. Exemplary current sensing schemesare shown in the circuit diagrams illustrated in FIGS. 2 and 3, whereFIG. 2 illustrates a PNP version and FIG. 3 illustrates a PMOS version.

Referring now to FIG. 2, the circuit includes a conventional foldedcascode amplifier portion (shown in dashed lines at 21) to provide highopen-loop gain to ensure the accuracy of close-loop gain (2×), theoutput power PNP device (Q1), and the current sensing circuit (shown indashed lines at 213). The circuit portion at item 25 provides the levelshift and driver for the base current of Q1 such that the base currentdoes not disturb the current matching in the folded cascode amplifier21.

For operation, the base current of the output power PNP Q1 is sensed byNMOS MN10. The current is then mirrored to MN11. In this example, thecurrent is mirrored by a factor of 15. The current of MN11 is the basecurrent of the sense PNP (Q2) in which the area of Q2 is made to 1/15 ofQ1. The emitters of Q1 and Q2 are both connected to VCC. For improvedcurrent matching, collect voltages Vc of Q1 and Q2 should be similar andtrack each other. NPN Q3 and PNP Q4 are added for this purpose, whereinthe Vbe of Q3 and Q4 are of the same amplitude but reversed signeffectively canceling each other. The equation as can be derived fromFIG. 2 is:V _(cQ2) =V _(cQ1) −V _(be1) +V _(be2) ≈V _(CQ1).and thus, Q1 and Q2 have approximately the same Vce. Thus, the collectorcurrent of sense PNP Q2 is 1/15 power PNP Q1 and the outputted sensedcurrent Isense is 1/15 that of the actual delivered current. In thisway, an efficient bipolar current sense scheme is developed that doesnot degrade the driver's performance to deliver power to the heatresistor Rheat.

For the case when an efficient vertical PNP is not available in thedesign process, the power PNP Q1 can be replaced by a PMOS in order toachieve high output voltage, as illustrated in the circuit shown in FIG.3. The circuit of FIG. 3 includes a conventional folded cascadeamplifier 31, output device PMOS M8, and sensing circuit 313. Theresistors at 35 are feedback resistors to form the feedback networks.

In FIG. 3, PMOS MP8 is the power output device. To sense the currentthrough the heat resistor, a sense PMOS MP9 is placed in close proximityto MP8. The gate of the sense PMOS MP9 is connected to the gate of powerPMOS MP8, such that they have the same Vgs. They also have the samesource voltage, which is VCC. In order to match the current well, thedrain voltage of these two PMOS should be similar and track thereforedevices MN14, MP14, MN11 and MN12 are added, wherein the voltagedifference between the drain of MP8 and the drain of MP9 is Vgsmn14 downand then Vgsmp14 up. MN11 and MN12 form a current mirror to ensureVgsmn14 is close to Vgsmp 14 in amplitude. From FIG. 3, the followingequation is derived:V _(dMP9) =V _(dMP8) −V _(gsMP14) ≈V _(dMP8).

The current mirror of MN11 and MN12 allows the current through MN14 totrack the current through MP14, so that V_(gsMN14)≈V_(gsMP14). In thisexample, the size of MP9 is held to approximately 1/40 of MP8. Thus, thedrain current of sense MOS MP9 is 1/40 of power MOS MP8.

Although exemplary embodiments of the invention are described above indetail, this does not limit the scope of the invention, which can bepracticed in a variety of embodiments.

1. A method for managing temperature of a device which receives variablepower for a first and second operation mode, the method comprising:determining power variance for said first and second operation modes;and determining a compensation power equivalent to said power varianceto a heater for increasing temperature of said device, said compensationpower is based on a delivery voltage, delivery current and resistance ofsaid heater; delivering a corresponding operation mode power; andcombining said compensation power with said corresponding operation modepower for providing approximately equivalent device temperature for eachof said first and second operation modes.
 2. The method of claim 1,wherein said first operation mode is a read mode and said secondoperation mode is a write mode, and wherein a write mode operationcurrent is greater than a read mode operation current.
 3. A method formanaging temperature of a device which receives variable power for afirst and second operation mode, the method comprising: determiningpower variance for said first and second operation modes; and deliveringa compensation power equivalent to said power variance to a heater forincreasing temperature of said device, said compensation power is basedon a delivery voltage, delivery current and resistance of said heater;said compensation power is cooperable with a corresponding operationmode power for providing approximately equivalent device temperature foreach of said first and second operation modes; and maintaining saiddelivery voltage at a constant voltage, wherein said delivery current isvaried corresponding to a variance in resistance of said heater.
 4. Amethod for managing temperature of a device which receives variablepower for a first and second operation mode, the method comprising:determining power variance for said first and second operation modes;and delivering a compensation power equivalent to said power variance toa heater for increasing temperature of said device, said compensationpower is based on a delivery voltage, delivery current and resistance ofsaid heater; said compensation power is cooperable with a correspondingoperation mode power for providing approximately equivalent devicetemperature for each of said first and second operation modes; anddetermining a resistance of said heater, wherein said heater resistancevaries with temperature; and adjusting said delivery current formaintaining said compensation power based on said determined resistance,wherein said delivery voltage is maintained constant.
 5. The method ofclaim 4, wherein said determining a resistance includes sensing acurrent received at said heater and determining said resistance fromsaid sensed current and said delivery voltage.
 6. The method of claim 5,wherein said sensing further includes providing a sensed current valuewhich is only a small portion of said current received at said heater.7. The method of claim 1, wherein said heater is provided in a heattransfer relationship with said device.
 8. The method of claim 1 furtherincluding determining heat variance of said device between said firstoperation mode and said second operation mode.
 9. The method of claim 1,wherein said device is a magneto-resistive head used for reading andwriting information to a magnetic media responsive to respective controlcurrents, wherein a determinable amount of heat is delivered to saidhead based on said control current and a resistance of said head.